It is known in the art to provide a pressure proportioning valve in a vehicle brake system having a master cylinder and front and rear wheel brakes. A typical proportioning valve limits the pressurization of the rear brakes relative to the front brakes in response to a master cylinder pressure that exceeds a predetermined minimum. During initial buildup of master cylinder pressure, the front and rear brakes have substantially equal pressure. When the master cylinder pressure exceeds a predetermined minimum, the proportioning valve becomes effective to limit further increases in the pressure at the rear brakes whereby the front-to-rear pressures are at a ratio of less than one-to-one. United States Patent No. 3,423,936 discloses such a proportioning valve.
A pressure proportioning valve interposed in a brake system functions to compensate for differences in the effectiveness of the front and rear brakes. The differences in effectiveness may be due, in part, to differences in the types of brakes employed (for an example, disc brakes on the front wheels and self-energizing drum brakes on the rear wheels) and also, for the dynamic weight shift to the front axle during a brake application. It is the weight shift which produces the principle need for a proportioning valve.
The imbalance between the effectiveness of the front and rear brakes is particularly apparent in a truck and varies substantially depending upon the amount of load which the truck carries. In a fully loaded truck, the weight on the rear axle may be such that the rear brakes having a braking capability that is almost as great as that of the front wheels. When the same truck is in an unladen condition, however, its rear brakes will not be nearly as effective as the front brakes. In such an unladen truck, equal pressure at the front and rear brakes would produce rear wheel skid when the master cylinder pressure is high enough to produce a moderate rate of vehicle deceleration.
A proportioning valve for a passenger automobile brake system is provided with a fixed split point, that is, a fixed point in the buildup of master cylinder pressure at which the proportioning valve becomes effective to limit or retard further increases in pressure at the rear brakes. Because the amount of load carried by a passenger automobile is not substantial in relationship to the total weight of the vehicle, the relative effectiveness of the front and rear brakes is not changed substantially by vehicle load changes. Therefor, a proportioning valve having a fixed split point is acceptable in passenger cars.
In a truck, however, where the relative effectiveness of the front and rear brakes changes greatly depending upon the load on the vehicle, it is desirable to have a proportioning valve with a split point that may be modulated in response to the amount of load and the effectiveness of the brakes. In accordance with the present invention, a proportioning valve is provided having inertia sensitive means for modulating the split point of a brake proportioning valve in response to the rate of vehicle deceleration and master cylinder pressure.
In a vehicle brake system, two inherent conditions exist that are compensated for by the valve of the present invention. These conditions are known as "dead time" and "vehicle lag". Dead time refers to the delay that occurs between initial pressure buildup and brake application at the wheels. During dead time, the brake shoes are moving from an at-rest position into contact with the brake drums. The second condition known as vehicle lag, refers to the delay or lag that occurs between the instant brake pressure is established and the time when the vehicle begins to decelerate. This delay results from the dynamic characteristics of the brake system and the vehicle. It is caused, in part, by elasticity in the brake system such as in the brake lines and in the suspension system which supports the vehicle wheels.
In the presently preferred embodiment of this invention, a brake valve is provided that includes (1) means for proportioning front to rear brake pressures, (2) inertia sensing means for modulating the proportioning function of the valve as a factor of vehicle loading, and (3) means for modifying the operation of the valve to compensate for dead time and vehicle lag.